This invention relates to probes for high resolution thermal analysis, and in particular a cantilever probe with an integral heating element.
The family of thermal analysis techniques, collectively called micro-thermal analysis (micro-TA), has been in existence for nearly a decade now. Micro-TA methods are based on a scanning probe microscope in which the conventional passive probe, typically a cantilever arm with integral tip, is enhanced to measure temperature and be resistively heated. This type of scanning probe microscopy (SPM) is called scanning thermal microscopy (SThM, specifically in this application, SThM in which the probe is actively heated rather than the sample). This form of microscopy allows thermal properties such as thermal conductivity and diffusivity to be mapped on a sub-micron scale. The heated probe will cause highly localized surface effects due to temperature. Used with an SPM, which is extremely sensitive to height variations, measured by changes in the deflection of the cantilever probe, heating the surface will cause cantilever deflections due to local thermal expansion or material softening or both. Additionally, the amount of power fed to the heater can be plotted independently or compared to the changes in cantilever deflection. Also, the power fed to two probes, one on the sample surface and one away from the sample surface can be compared to create a differential signal. The differential signal is used either (1) to produce localized analysis plots versus temperature that provide calorimetric or temperature dependent information at a specific position on the sample, or (2) to construct an image whose contrasts represent variations in thermal conductivity and/or diffusivity across a scanned area.
The general acceptance of this technique has been greatly hampered by the commercially available thermal probes having a tip diameter in the order of 3 microns which restricts the spatial resolution to this regime. Currently available Micro-TA probes are typically fabricated from fine wire, and the sharpness that can be achieved is limited. There are several fields, the two most prominent of these being Semiconductors and Polymers, where a spatial resolution of sub 500 nm will give information to scientists that could lead to revolutionary breakthroughs in scientific understanding and corresponding technology breakthroughs.
Probes having tips sharper than those made from fine wire can be produced from silicon, silicon nitride and other materials using microfabrication techniques such as used to make conventional SPM cantilever probes. Heating elements can be added to the cantilever. The heating element, which can be heated through resistive, inductive, or other heating approaches, may be produced on a cantilever body in a number of ways. One approach, described in U.S. Pat. No. 6,932,504, is to deposit metal film traces, such as used in normal semiconductor manufacturing for interconnects. This approach does not allow for suitably high temperatures. The low resistance of most metal films will require a significant current in order to achieve any reasonable heating. If there are any defects in the film or any contact of the film to any other conductive surface the heating element can fail even at low temperatures and at higher temperatures the element will likely fail due to alloying, electro-migration, melting, or other failure mechanisms.
Another approach, used by researchers at IBM and other laboratories (see IEEE Journal of Microelectrical Mechanical Systems, Vol. 7, No. 1, March 1998, pp 69-78) is to dope portions of the cantilever using an ion implant process. The doped regions become electrically conductive. A high doping can be used for conductive traces which connect to a heating element. The heating element can be doped at a lower level, resulting in a higher resistance, thus constraining the resistive heating to just the area of lower doping. Such an approach produces heaters that can achieve very high temperatures on probes with sharp tips. The doping process lends itself well to manufacturing the cantilever using conventional microfabrication processes, as implanting is a common step in silicon processing. However all current examples of thermal probes with implanted heating elements have the probe tip and the heating element overlap. This, for reasons that will be explained below, results in a very short, typically 1 micrometer or less in length, tip. Such a short tip is suitable for some applications, but in general, use of such a short tip constrains the probe to interrogating surfaces with topography having roughness less that the length of the tip, thus unduly limiting the application of the technique. Another benefit to having a longer tip is that the technique can be used to measure film thicknesses and thermal properties of thin films or multilayer films. The probe, if sufficiently long can melt through one layer and interrogate a layer below.
Thus the object of this invention is the design of a probe that can perform nanoscale thermal analysis with a spatial resolution of less than 500 nm and is applicable over a wide range of sample surface topography.